Notchl's oncogenic activity in T cell progenitors appears to represent an exaggeration of its normal functions during T cell development. We will use new ChlP-Seq and bioinformatic technologies to delineate the interaction of Notchl with the genomes of normal murine and human thymocytes. By correlating these interactions with chromatin marks and gene expression, we will gain a global view of how Notchl regulates T cell development, and by comparing these interactions with those of Notchl in murine and human T-ALLs, we will further gain a deep understanding of key similarities and differences between normal and malignant thymocytes. A second key aspect of Notchl interaction with normal and malignant thymocytes is regulation of gene expression through sequence-paired binding sites (SPSs) for the transcription factor CSL that permit Notchl dimerization. Mutants that disrupt dimeric Notch complexes cannot induce T-ALL, show defects in T cell development, and lose the ability to upregulate key target genes such as Myc and pTa. These complementary lines of investigation will be pursued through two aims: Aim 1; To determine how Notchl regulates T cell development. We will combine ChlP-Seq with computational approaches to identify Notch1/CSL binding sites genome-wide, characterize the specific response elements that control transcription of key Notch target genes, identify both novel Notchl target genes, and elucidate mechanisms used by Notch to regulate p-selection and other stages of T cell development. In addition, the epigenetic landscapes of normal stages of T cell development will be compared to T-ALL cells. Aim 2: To determine the role of dimeric Notch signaling complexes in T-ALL. We will identify and validate dimerization-dependent Notch targets and determine the in vivo importance of dimerization-dependent Notch signaling during T cell development. Together, these studies will provide a comprehensive molecular and genomic understanding of how Notch regulates T cell development and T cell transformation, and in doing so provide new opportunities to rationally target the Notch pathway in T-ALL and other diseases.